That is one. But how, seduce a partner stronger than oneself?
oh you can get real creative with it. Certain race of people that grant magical powers from sex. And then I suppose lets say those powers can be passed on by sex (eventually becoming diluted and bad, making that generation have to fuck them again). IF the powers don't pass on, I guess the race can be like put into each town and people from nearby go there to fuck them for their powers. Although I guess that's kind of evil. they will escape. So we, as the author, then make it so they cant use their abilities. And they are immortal. With their one purpose in life being sex.
or the parent pass it to the offsprings by... well... sex.
And of course, your idea can be applied too. Lets say there existed a race that passed down magic to weaker humans by sex, but they couldn't actually reproduce and eventually got extinct when their lifespan got over. However, during that, the human bodies got used to a higher being pushing in their magic and evolved to gain magical abilities that way, maybe the race knew of it and banned it since it was sacrilegious. But after they went extinct, the humans started bestowing magical abilities amongst themselves by sex.
Alternatively, Maybe the race fucked only the starting humans and they passed down their magic to weaker humans by sex ever since. You can also separate the abilities into tiers. Maybe each tier gives you further insight into magic and allows you to wield it more efficiently. And of course, to advance through this tier, you have to fuck stronger, higher tier humans.
You can balance this out by making it so that they cannot fuck over and over again, this should produce a threat of extinction that needs to be resolved in the story. Easily giving you an antagonist in the very beginning. The law of nature itself.
You can also go simple, adam and eve but weird route, where god fucked two people and they got magical powers, then those two fucked each other and the powers passed on. But that's boring!
A better, simple sex route will be that whenever someone gets of certain age (or when their born....) an eldritch-esque god fucks them and they gain magical powers.
This however, can quickly get really weird. But who cares about that? the only limitation is your imagination!